Vim youcompleteme not working in Debian Jessie

I contacted the maintainer and finally got the missing hint. As it turns out, all I had left to do was to enable filetype recognition. In case someone else struggles with this, here are some instructions on how to setup Vim with the Debian package vim-youcompleteme.

Install vim and vim-youcompleteme packages

$ sudo apt-get install vim vim-youcompleteme

This will also install any required dependencies automatically. Afterwards you will have to issue the following command

$ vam install youcompleteme

The last command will create ~/.vim folder in your home folder. However, code completion does not work yet. Some further modifications are required. First copy the default ycm_extra_conf.py file to the newly created ~/.vim/ folder

$ cp /usr/share/doc/vim-youcompleteme/examples/ycm_extra_conf.py ~/.vim/.ycm_extra_conf.py

Then we need to tell vim to use this file for code completion in our .vim.rc and to turn on filetype recognition. The following two lines should be added to ~/.vimrc:

let g:ycm_global_ycm_extra_conf = "~/.vim/.ycm_extra_conf.py"
filetype on

Now vim should be able to use code completion, e.g., with C++ files.